Capacitor type differential pressure switch



July 3, 1956 E. J. RICKNER CAPACITOR TYPE DIFFERENTIAL PRESSURE SWITCH Filed Sept. 3, 1952 .w wf

mw wwf W M M /f United States Patent O CAPACITR TYPE DIFFERENTIAL PRESSURE SWITCH Edward J. Rickner, Glenside, Pa. Application September 3, 1952, Serial No. 307,729 1 Claim. (Cl. 323-74) (Granted under Title 35, U. S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a pressure responsive control system and to a control switch for such a system.

It is Often necessary in certain applications of fluid pressure systems to maintain the pressure of the fluid at a predetermined constant pressure or within a certain prescribed diiferential from a reference pressure. In prior applications the pump or other means which maintained pressure of the working uid was controlled by means of mechanical linkages connected to a diaphragm which responded to changes in the reference pressure or to differentials in pressure between the reference fluid and the working fluid.

Mechanical systems had the disadvantages that the sensitivity, stability and precision of adjustment possible were critically limited by the mechanical precision of the parts and when the system was subject to acceleration forces, for example, in airplane installations, the inertia of the parts affected the operating point of the control.

The mechanical linkages were generally connected to open and close electrical contacts and here again serious difficulties were encountered due to wear of the contacts, the formation of films or dirt layers on the contactsr and friction and sticking of the parts which affected the operating point and the pressure differential required to operate the mechanism.

The salient features of applicants invention are the provision of a pressure sensing unit which converts a change in pressure or in pressure differential to a corresponding change in electrical capacitance which in turn causes the appearance of an unbalance voltage in a bridge circuit, the elimination of any mechanical connections between the diaphragm and the electrical contacts, the use of an electronic circuit to amplify the output of the bridge circuit and the utilization of a phase differentiating electronic circuit to energize a relay according to the phase and amplitude of the amplified voltage and thus close the electrical contacts which control the pump or permit the contacts to remain open.

lt is an object of this invention to provide a differential pressure switch incorporating the features set forth above.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

Figure l is a schematic representation of the pressure sensing unit and the associated electronic circuits which constitute the invention;

Figure 2 is a plan view of the pressure sensing unit and Figure 3 shows an enlarged cross-section of sensing unit taken on line 3-3 of Figure 2.

The structure and operation of the switch will be explained in conjunction with Figure l. 50 is a transthe pressure former, the primary winding 51 thereof being connected to a source of alternating current voltage. The secondary 61 of the transformer has connected across its central portion at 54 and 55 a potentiometer 59 whose movable contact 58 is connected to ground. Thus a voltage V1 appears between one side 52 of the secondary winding and ground and a voltage V2 appears between the other side 53 of the secondary and ground. The ratio of voltages V1 and V2 may be changed by moving the center contact 58 of the potentiometer for a purpose to be hereafter explained. Each side of the secondary winding is connected to a separate circular disk 12 and 15 enclosed in a relatively at cylindrical housing 10 which has a port at one side to admit a pressure P1 and at the other side to admit a pressure P2. A thin flexible disk or diaphragm 17 is located in the housing between the disks 12 and 15 and forms a pressure tight seal hetween the two chambers thus created. The disks 12 and 15 with the diaphragm 17 form two capacitors which constitute the capacitance arms of a capacitance bridge circuit. The capacitors are in series across the voltage source and the voltage drop Ver and Vcz in each depends upon the capacitance C1 and C2 of each. The capacitances depend upon the relative position of the diaphragm with respect to each of the disks and the position of the diaphragm depends in turn upon the relationship 0f the pressures P1 and P2. Theoretically the diaphragm should assume a position in which it is equally spaced from disks 12 and 15 when P1 equals P2 and a balance condition wherein no voltage V0 exists between diaphragm 17 and ground would occur when P1 is equal to P2 and the potentiometer is adjusted to make V1 equal to V2. In practice, however, may be closer to one or capacitances of the conditions do not follow the theoretical.

the other of the disks and the In order to compensate for the deviation of the actual4 from the theoretical, the circuit is balanced as follows:

with P1 equal to P2 the voltages V1 and V2 are adjustedpotentiometer until Vo equals zero. The: may thus be balanced without the necessity' of assuming the task of critical mechanical construction:

by means of the bridge circuit of the capacitor.

lt can be seen that by following the balancing procedure outlined above the voltage Vo may be made zero when a predetermined pressure differential P1 minus P2 exists. Thus the switch may be set to keep the pressures P1 and P2 equal to each other or to maintain a predetermined pressure differential P1 minus P2.

With the circuit in a balanced condition in which P1 equals P2 or with a given pressure differential P1 minus P2 a change in one of the pressures would cause the diaphragm to be deflected toward one of the disks 12 or 15. This would cause a change in the values of capacitances C1 and C2 accompanied by a change in the distribution of voltages between the capacitors and would result in the appearance of an output voltage Vo between the diaphragm and ground.

The output voltage Vo is carried through several stages of amplitication and impressed on the grid 7i of a thyratron tube 68 which is normailyin nonconducting condition. Applicant employs the two sections of a 12AX7 vacuum tube in cascade for amplification and carries the signal to the grid ofthe first section by means of coaxial cable 22. A capacitor 62 is connected in parallel with the grid resistor 63 and changes may be made in its value to make slight changes in the phase of the voltage reaching the amplifying stages. Applicant successfully uses a .003 microfarad capacitor and a 1.5 megohm resistor with a 400 cycle signal voltage.

Direct current plate supply voltage for the amplifiers is provided from a rectifier 77 and associated filter circuit.

since the diaphragmA the two capacitors may not be identical For example, applicants circuit employs a 6 X 4 vacuum tube connected to the 400 cycle source. A band pass lter comprising a capacitor 67 and a variable inductor 66 is.c,onnected.in .the plateycircuit of. the nal-sta'ge'of ampli.LA cation A withv the inductancex set'; to: passa the. frequencyA off the supply; voltage. Applicans.circuit-designed for au 400'cycle supply,y utilizes a; .0i-l microfarad; capacitor, andi an-inductance variablefronr.1lI to.; 39- henries. Thexfilter circuit' prevents the passage-0f noise' or harmonicsv ontothe` thyratron grid thereby providing: greatersensitivity and stability. of operation. Sfupplyfvoltages. for the thyra.- tron are vprovided fromtthesameialternatingcurrent:source a leadY connected: tothe: peint 60. Bias. resistors` 72'.' andI 7 3' are provided in; the; cathode circuiti to; keep.- the 2D2 li tube used by applicantiirr4 the nonconductingk region whensthereisnogridrsignalz.` WhenA apositive signal isimposed on thezgrid. 7.11 coincident-:withthe occurrence of a positive voltage ontheplatefl andif the signalivoltage is ofA sufcient amplitudewith respect tothevoltages-on theglate andf the cathode70; theitube. willfre andwill continuetoccnduct.further remaining portion .offthe positive,- cyclefA plate voltage. The; output current ofA the thyratron will be a;;serie sof positive halff-cyclesresembling theunltered output; of;v a half-wave rectifier This cur-- rent passesthrough the coil 7S- of a relay whichihascon-V trolcontacts 7.6, that openror close a circuitfor a-motor-orA pump or other; devicefused-,to maintainA the working pres surefPzL Acapacitor-74 in parallel withthe coil 75 acts to provide anv average direct'currentfV through the coil or winding: It; will. be. seen that ifthevoltage Vo onl the@ grid7Lis'of alrelatively low amplitudeaorl isdisplacedi in phase; from the plate.- voltage on the thyratron= sol thatf Conductionwill takegplace only during-a small part ot theVv positive half-cycle of the-plate-` voltage, theaverage currentowingthrough the winding 75 will-be insufli'cientVA to; operate; the; relay. and contactsA 76' will remain open. 'Ifhereforeit isevident'that control of the switch is dependentmot only upon the amplitudeof the signal voltage; but on the phase as` well.y Thus, should the differential pressure.- Pr-Pz fall` below the value determinedrbythe setting of the'movabl'e contact ofthe-potentiometer59; avoltagewouldappear on thegridoffthe amplifier. andvafterl amplificationwouldappear on the grid71f of thethyratron; However, it will be noted tljatithis voltage would heapproximately 180' out. of phase withl the plate voltage of tube 68 so that conduction would not takeV place;

Thei electronic circuits are shown by way of example only'and" it is' obvious that various othercircuitarrangements vmay be utilizediwith the pressure sensing unit'and` bridge; circuit disclosed:

By eliminating mechanical connections to the Vpressure sensitive diaphragm and making the` operation of' the switch depend insteadon Vanelectrical potential sensitivity of the: switch remains fairly constant over a wide range: ofoperatingconditions. In addition the degreeofiineness of. adjustment possible is' greaterwith the potentiometer. than with a mechanical'arrangementand adjustmentsV may be made fromla-position remote from the-location ofthe switch itself. The switch willoperate eciently at'very low-pressures with good stability andsensitivity, for eX'-. ample, the device was successfully used to'operate'fan air mileagel pump to maintain` pressure equal to that of an external source at pressuresranging from 0.00009 pound" per square inch to-0.24 poundper square inch.

Several important features of the pressure sensinglluit. will become apparent in the following description of the mechanical'details thereof shown inFigures 2 and 3.

The pressure sensing unit comprises ay relativelyA at cylindrical rmetalvhousingycomposed of two parts 10 `and 1 1 fastened together by screws 37- threaded'intoYtheporu tion I 11. of the housing. Theassembled; housing includes,

17.? Whih; consttutes the;A common element; of;` the' two"- 31. capacitors.. isprovidednear. its outer edgewith.. anannular` reinforcing ring 33 which is clamped between the two sections of the housing and is insulated therefrom by annular insulating gaskets 34 and 35 on either side of the reinforcing ring. Sleeves 36 of insulating material are provided around each screwfifl to insulate the ring 33 from said.screws.and from-.the housing, Flatrnetallicdisk-s' 12 and'15`attaclied to the liousingby circumferentially spaced screws 29 and 30 respectively form the other conductor for each of thebrid'ge'circuit: capacitors. Deflection of the diaphragm is limited by the disks 12 and.15so-thatdistortion of `the diaphragm under extreme conditions of pressure unbalance isrpreventedl To prevent shorting of the capacitor under, such. conditions,. an insulating coatingv 13 and 16 is applied to each disk on its face nearest the diaphragm. Each of the disks 12 and 15 is backed by spacers 25 and 26 and insulating washers 2,7 and. Z8. respectvely'placed: around" each of the screws 29' and 30. Each-of' the screws-'29" and! 310 is insulated from the housing where'itpasses'therethrough by bushings 23 and 24' respectively and is provided` with a retaining nut and washerl', 32E External electricalconnection to the disks 12 and 15 isma'dethrough'connector lugs 81 and 82 and one'ofthecscrews 29 and 30'. A central bore 20 in part 10 and a similar` borev 2-1 in part 11 can be provided with a fitting, to carryjthe pressures: Pr and P2 respectively into the chamber-` containing the diaphragm.

Part* 11 of the housing is provided with a bore 39 through which passes a pin 38' electrically connected" toY the; diaphragm 17; The pin passes through the gasketSSv and: ispressed'intoy the reinforcingl ringY 33. The free endY ofthe pin projects into a chamber 7S formed in a block. 40 xed to part 11 and is, connected by means ofganinsulatedwireto connector pin 43 of a coaxial ,cable conf nector' 422; The element 41' is an insulator disk. in. the cable1 connector;

TheV housing is grounded to theV common. electricalv ground for the bridge circuit and electronic circuit and effectively'shields the capacitors fromelectricaliields outsideithe housing. All'lines of force between the disks 12 and' 15- andthe diaphragm must pass throughrthe huid-in the system, there, being no complete path of solid dielectric. for` lines of force passing between said disks and da: phragm. This enhances the sensitivity and stability characteristics ofthe switch when air is the Huid since it has more stable dielectric characteristics than practicable known solids.

Obviously many modifications and variations of the presentinvention are possible in the light ofthe above teachings; It is therefore to be understood. that within the scopej ofthe appended claim the inventionmay bepracticedotherwise than. as specifically described.

What is claimedis:

In afpres'sure responsive switch, a pressuresensing unit comprising a relatively at cylindrical housing formed from twomating separable sections,Y a thin metallic diaphragmclamped between the two sections of the housing dividing the housing intotwo, chambers, each having a port to admit a gaseous fluid; an annular gasket on each side ofthe diaphragm to insulate it from the housing? a circular metallic disk in each of said chambers' forming with said" diaphragml a, p air. of. capacitors,y means attachingcsaidldisksr to the housing. and insulatingv themthere.- from, a .rst. source: ofvoltage-providing a. potential; with; respect to ground toone. of said disks,.a second sourcel ofcvoltageprovidingapotential with respect to groundr to theotherl disk', means insulated from the housingfor con necting said sources of voltage to their respectivedisks and means :forf adiusting the-iiist andV second` voltages so that a predetermined pressure diiferential may exist betweenf the-two chambers" of; theA unit; said housing being connected to groundsothat no` complete path oft' so1id` delectric'exis'tsffor lines-of^'fo1ceA from the diaphragmto either-ofthe disks,- withathe maximum deection ot'the`` diaphragm being limited by Contact of the diaphragm with 2,525,016 one or the other of said disks. 2,538,881 2,547,780 References Cited in the file of this patent 2,567,253 UNITED STATES PATENTS 2,577,435 2,092,762 Kroger Sept. 14, 1937 2696566 2,261,364 Grove Nov. 4, 1941 2,347,590 Binder Apr. 25, 1944 2,384,202 Smith sept. 4, 194s lo 21 2,522,574 Hagenbuck Sept. 19, 1950 Borell Oct. 10, 1956 Schommer Jan. 23, 1951 Reynst Apr. 3, 1951 Strange Sept. 11, 1951 Siebenthaler Dec. 4, 1951 Lion Dec. 7, 1954 OTHER REFERENCES Kampinsky: Abstract of application Serial Number 9,100, published March 18, 1952, 656 O. G. 894. 

